Pollen morphological support for the Catesbaeeae-Chiococceae-Exostema-complex (Rubiaceae)

The pollen morphology of the Catesbaeeae-Chiococceae-Exostema complex as recently treated by Delprete (1966) was examined with LM and SEM. The group is remarkably stenopalynous; typical representatives have medium sized, 3-colpate pollen with a perforate tectum covered with microspines. The inner nexine ornamentation is pronounced and offers more variation than the sexine pattern. A typology of the inside structures is presented based on LM observations and SEM observations of sectioned grains. Orbicules are common and numerous in the Catesbaeeae and Exostema-group; for most genera of the Chiococceae confirmation is needed of orbicule presence. All orbicules observed are relatively large (1-4 μm) and spiny. Pollen and orbicule morphology proved to be a powerful tool to delimit the Catesbaeeae-Chiococceae-Exostema complex. The overall delimitation of the complex is corroborated with our pollen data. The genera Mastixiodendron and Placocarpa, however, can be excluded from the complex based on their pollen morphology. Mastixiodendron has 3-colporate, perforate pollen without microspines and the endocolpi are fused into an endocingulum. Pollen of Placocarpa is reticulate and 3-colporate with perpendicular endocolpi.